It is proposed to continue the study on DNA damage caused by carcinogens; the subsequent repair of such damaged DNA and replication of carcinogen-damaged DNA in vivo, with emphasis on two aspects - (a) the relationship between the qualitative nature and intragenomic distribution of carcinogen-DNA interaction and DNA repair; (b) a detailed study on the in vivo replication of DNA segments with different types of damage. Increased attention will also be placed on developing our method on "detection and determination of DNA damage and repair in vivo" into a highly reliable bioassay for carcinogens.